We demonstrated previously the specific in vitro killing of murine tumor cells in the presence of lymphoid cells from non-sensitized syngeneic mice and decomplemented sera from mice bearing the particular tumor (Moloney virus - or 3-methycholonthrene induced sarcomas or mammary tumor virus-induced adenocarcinomas). Cytotoxicity does not occur in the presence of either the serum or lymphoid cells alone. This antiserum-dependent cellular cytotoxicity (ADC) can be demonstrated in a microcytotoxicity assay by (a) pretreating plated tumor cells with serum prior to addition of the effector lymphoid cells and by (b) "arming" the lymphoid cells with serum and adding the armed cells to the plated target cells. We propose to study (a) the nature of the arming factor by using affinity chromatography to specifically deplete different classes of immunoglobulin from the sera; (b) the nature of the effector lymphoid cells by use of anti-theta serum to remove theta-bearing T cells and of anti-immunoglobulin affinity columns to remove Ig-bearing B cells; (c) the potential use of armed cells for immunoprophylaxis and immunotherapy in vivo.